Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common. One such plate is available from Eastman Kodak Company as the KODAK Direct Image Thermal Printing Plate. It includes an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
Dry planography, or waterless printing, is well known in the art of lithographic offset printing and provides several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste. Silicone rubbers, [such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes)] have long been recognized as preferred waterless-ink repelling materials. The criteria for waterless lithography and the ink repelling properties of poly(siloxanes) have been extensively reviewed in the TAGA Proceedings 1975 pages 120, 177 and 195 and 1976 page 174. In addition to low surface energy, it was concluded that the ability to swell in long-chain alkane ink solvents (i.e., its "oleophilic" nature) accounts for silicone's superior ink releasing characteristics. An important consideration is that siloxane polymers repel ink.
In the lithographic art, materials that release or repel oil based inks are usually referred to as having "oleophobic" character. Herein, ink repelling materials are defined as "melanophobic" and, conversely, the term "melanophilic" is used to describe ink "loving" or accepting materials.
The basic method of preparing a waterless printing plate involves the imagewise removal of silicone to expose an underlying ink accepting surface. For example, U.S. Pat. No. 3,677,178 (Gipe) discloses a waterless lithographic offset printing plate having a flexible substrate overcoated with a diazo layer that was in turn overcoated with silicone rubber. The plate was exposed to actinic radiation through a mask, initiating a reaction in the diazo layer that rendered the exposed areas insoluble. Development was accomplished by swabbing with a cotton pad containing water and a wetting agent to remove the unexposed coating areas.
It was recognized thereafter that a lithographic printing plate could be created containing an IR absorbing layer. Canadian Patent 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose) and an optional cross-linkable resin. Such plates were exposed to focused near IR radiation with a Nd.sup.++ YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing, or ablating the absorber layer and the overlying silicone rubber. The plate was developed by applying naphtha solvent to remove debris from the exposed image areas. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. CO.sub.2 lasers are described for ablation of silicone layers by Nechiporenko & Markova, PrePrint 15th International IARIGAI Conference, June 1979, Lillehammer, Norway, Pira Abstract 02-79-02834.
More recently, WO 94/18005 discloses the use of dry cotton pads or non-solvent wiping to develop dry planographic plates after laser imaging.
Direct digital imaging on-press or a platesetter is also well known. In this case, the printing plates having various layered structures wherein the layers having different affinities for ink and printing liquids are exposed to ablative absorption on press to create a printable lithographic surface in response to digital information supplied to a laser imaging apparatus. See, for example, U.S. Pat. No. 4,718,340 (Love III), WO 92/07716 (Landsman), U.S. Pat. No. 5,379,698 (Nowak et al), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,351,617 (Williams) and U.S. Pat. No. 5,353,705 (Lewis et al). In using these technologies, removal of the silicone rubber after exposure requires a development step that includes wiping.
Due to the toughness and thermal stability of crosslinked silicone polymers, printing plates containing same are limited in their reproducibility of the images when laser ablation of the polymers is used for imaging. The problem arises from the conflicting need to have wear resistant silicone polymer layers for long press runs while maintaining ease of layer removal by laser ablation. Crosslinking makes complete removal more difficult, and silicone polymer debris clings to the underlying layers, and must be physically wiped off, as noted above. Wiping presents several disadvantages, including the difficulty of reproducibly removing all debris, and the susceptibility of the printing plate surface to scratching during wiping or other mechanical cleaning operations.
The need to change the nature of silicone layers has been recognized. For example, U.S. Pat. No. 4,755,445 (Hasegawa) describes the use of photohardenable microcapsules in a "waterless" printing plate. After imaging, unexposed microcapsules are broken, releasing an ink-receptive compound onto the silicone surface. This approach suffers from the need for a second UV exposure or heating step to complete the plate image, and is not suitable for direct digital imaging.
JP Kokai 60-196347 (Toray industries) describes "painting" a silicone plate surface with ammonium fluoride to etch away the silicone surface, followed by washing. The ammonium fluoride can also be applied in a polymeric dispersion using various techniques. Subsequent heat treatment adhered the polymer to the silicone surface. This imaging system and method are cumbersome and complicated, and make it difficult to produce fine details on a printing plate.
There is a need for processless, digitally imageable printing plates, that have high writing sensitivity (requiring low laser energy for imaging), excellent image quality, and long run length. Such imaging members must have a tough surface silicone layer, but must be easily imaged with minimal debris in background areas without wiping or any other mechanical cleaning process.